BR112019010870B1 - HIGH MN STEEL PLATE AND MANUFACTURING METHOD FOR THE SAME - Google Patents

Abstract

It concerns a high Mn steel plate and a manufacturing method for the same. The high Mn steel plate has a component composition containing, wt%, C: 0.20 to 0.70%, Si: 0.05 to 1.0%, Mn: 15 to 30%, P: 0.028 % or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.5 to 7.0%, Ni: 0.03 to 0.30%, N: 0, 0010 to 0.0200%, and one or two or more of Nb: 0.003 to 0.030%, V: 0.03 to 0.10%, and Ti: 0.003 to 0.040%, with the balance being Fe and incidental impurities, in that: a 0.5 mm microstructure under a steel plate surface includes austenite as a base phase; and 25% or more of the austenite, by area ratio, has an equivalent circle diameter of 10 µm or more and an aspect ratio of 1 major axis to a minor axis of 3 or more.

Description

FIELD OF TECHNIQUE

[0001] The present invention relates to a high Mn steel plate that is suitable for structural steel used in a cryogenic environment, such as a liquefied gas storage tank, in particular, to a high Mn steel plate which has excellent resistance to stress corrosion cracking in a corrosive saltwater environment and to a manufacturing method therefor.

TECHNICAL BACKGROUND

[0002] When a hot rolled steel plate is used for a liquefied gas storage structure, cryogenic temperatures are encountered in the environment in use. Thus, for cryogenic temperatures, the steel plate is required to have not only strength but also toughness. When hot rolled steel plate is used for storage of liquefied natural gas, for example, excellent toughness at the boiling point of liquefied natural gas of -164°C or less is required. When the low temperature toughness of a steel material is poor, there is a risk of failure to maintain the safety of a cryogenic storage structure. Consequently, there is a high demand for improved low temperature toughness of a steel material to be used. In response to such a demand, 5000 series aluminum alloys, 9% Ni steel, or austenitic stainless steel that includes, as a steel plate microstructure, austenite that does not exhibit brittleness at a cryogenic temperature have been conventionally used. However, since its alloying costs and/or manufacturing costs are high, there is a need for an economical steel material that has excellent cryogenic toughness. Consequently, as a new steel material that can replace conventional cryogenic steel, the use of a high-Mn steel plate, to which a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, is added, has been investigated as structural steel. for a cryogenic environment.

[0003] Meanwhile, when austenitic steel is used in a corrosive environment, austenite grain boundaries are eroded and there is a problem that stress corrosion cracking tends to arise under applied tensile stress. Especially, in a manufacturing stage for a liquefied gas storage structure and the like, an iron base surface of a steel plate is exposed in some cases. Upon contact of a surface of steel material with oil, moisture, and/or water vapor containing a corrosive substance such as salt, corrosion of the steel material arises. A high Mn steel plate that has been conventionally investigated has, in some cases, lower corrosion resistance than 9% Ni steel and common low alloy steel, not to mention austenitic stainless steel. In corrosion reactions on the surface of such a high-Mn steel plate, oxides (rust) are formed by the anodic reaction of iron while hydrogen is generated by the cathodic reaction of moisture, thereby promoting stress corrosion cracking due to hydrogen embrittlement. . Consequently, stress corrosion cracking can result in failure of a structure in the presence of applied stress in the environment in use or in the presence of residual stress through bending or welding during fabrication. Consequently, in view of safety, it is important to have excellent resistance to stress corrosion cracking, not to mention strength and cryogenic toughness of a steel material to be used.

[0004] Patent Literature 1, for example, discloses a steel material that has improved machinability and Charpy impact characteristics at -196°C in weld heat affected zones by adding 15 to 35% Mn, 5 % or less Cu, and appropriate amounts of C and Cr.

[0005] Furthermore, Patent Literature 2 discloses a high Mn steel material which has improved low temperature toughness through addition of C: 0.25 to 0.75%, Si: 0.05 to 1.0% , Mn: more than 20% and 35% or less, Ni: 0.1% or more and less than 7.0%, and Cr: 0.1% or more and less than 8.0%.

CITATION LIST PATENT LITERATURE

[0006] PTL 1: Japanese Unexamined Patent Application Publication (PCT Application Translation) No. 2015-508452

[0007] PTL 2: Japanese Unexamined Patent Application Publication No. 2016-84529

SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0008] The high Mn steel plates disclosed in Patent Literature 1 and 2, however, are intended to have low temperature strength and toughness. Charpy impact characteristics at -196°C in weld heat affected zones are 60 to 135 J (disclosed only in Patent Literature 1). However, the cryogenic toughness of base metals is still unsatisfactory without achieving both cryogenic toughness and stress corrosion cracking resistance.

[0009] In view of the above problem, an object of the present invention is to provide a high Mn steel plate that has excellent stress corrosion cracking resistance and cryogenic toughness and to provide a manufacturing method for the same.

SOLUTION TO THE PROBLEM

[0010] To achieve the above-mentioned object, the present inventors have intensively studied, for high Mn steel plates, various factors that determine the microstructure, a manufacturing method, and a component composition of a steel plate to ensure excellent strength to stress corrosion cracking, and find the following.

[0011] 1. To achieve both cryogenic toughness and excellent resistance to stress corrosion cracking, it is effective to decrease the amount of hydrogen penetrating a steel plate through corrosion reactions. Improving the corrosion resistance of a steel plate surface in a saltwater environment is essential. For this purpose, it is important to strictly control the component composition of a high Mn steel plate as a base. In particular, by adding Cr and Ni together and properly controlling the amounts, the rust formed in the early stage of corrosion reactions on a steel plate surface becomes thin. Furthermore, by delaying subsequent corrosion reactions, the amount of hydrogen penetrating the steel can be decreased.

[0012] 2. Furthermore, it has been found that strictly controlling the microstructure near a steel plate surface is also effective in improving resistance to stress corrosion cracking. In other words, to improve stress corrosion cracking resistance, it is important that 25% or more of austenite, by area ratio, has an equivalent circle diameter of 10 μm or more and an aspect ratio of one major axis for a minor axis of 3 or more. This is presumably because hydrogen that has penetrated a steel plate through corrosion reactions is trapped within grains of non-recrystallized austenite, and consequently, the amount of hydrogen at austenite grain boundaries is decreased relatively, thereby decreasing the susceptibility of austenite grain limits for stress corrosion cracking.

[0013] 3. In addition to the above described 1 and 2, a carbide, a nitride and a carbonitride complex of Nb, V and/or Ti in a steel plate can further improve the resistance to stress corrosion cracking through control appropriate from its dispersed state. Such a Nb, V and/or Ti complex carbide, nitride and carbonitride act as diffusible hydrogen trapping sites in a steel plate. In other words, complex carbide, nitride and carbonitride act as trapping sites of diffusible hydrogen generated through corrosion reactions of a steel material and have an effect of suppressing stress corrosion cracking. The dispersed state of a carbide, a nitride and a carbonitride of Nb, V and/or Ti within austenite is affected, for example, by heating, rolling and cooling conditions in a hot rolling step. Consequently, it is important to control these manufacturing conditions.

[0014] 4. In addition, to effectively suppress the intergranular fracture of austenite, measurements to improve grain boundary strength are effective. P is an element that tends to undergo cosegregation with Mn in an ingot solidification process and decreases the strength of grain boundaries that cross zones of microsegregation. Consequently, it is necessary to decrease impurities such as P.

[0015] The present invention was made on the basis of the findings described above and further further investigation and is summarized as follows.

[0016] [1] A high-Mn steel plate that has a component composition containing, % by mass, C: 0.20 to 0.70%, Si: 0.05 to 1.0%, Mn: 15 to 30%, P: 0.028% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.5 to 7.0%, Ni: 0.03 to 0. 30%, N: 0.0010 to 0.0200%, and one or two or more of Nb: 0.003 to 0.030%, V: 0.03 to 0.10%, and Ti: 0.003 to 0.040%, with the balance being Fe and incidental impurities, wherein: a 0.5 mm microstructure under a surface of the steel plate includes austenite as a base phase; and 25% or more of the austenite, by area ratio, has an equivalent circle diameter of 10 µm or more and an aspect ratio of 1 major axis to a minor axis of 3 or more.

[0017] [2] The high-Mn steel plate according to [1], wherein the component composition further contains an element in at least one group selected from the following group A or B.

[0018] Group A: one or two, % by mass, selected from Mo: 0.05 to 2.0% and W: 0.05 to 2.0%

[0019] Group B: one or two or more, % by mass, selected from Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0010 to 0 .0200%.

[0020] [3] The high Mn steel plate according to [1] or [2], wherein the microstructure of 0.5 mm under the surface of the steel plate still includes a total number of 2x102/mm2 or more than one carbide, nitride and carbonitride, the carbide, nitride and carbonitride containing one or two or more of Nb, V and Ti and having an equivalent circle diameter of 0.01 to 0.5 μm.

[0021] [4] A manufacturing method for a high Mn steel plate, which includes: heating steel having the component composition according to any one of [1] to [3] to a temperature range of (Tx-50) °C or greater and (Tx+200) °C or less as a steel surface temperature for any one or more of Tx (°C) defined by any one of formula (1) to (3) when Tx (x = Nb, V or Ti) is defined at a temperature represented by any one of formulas (1) to (3); hot rolling at a finishing temperature of 750°C or greater and 1000°C or less to produce a steel plate; and subsequently cooling at an average cooling rate of 1.0°C/s or more on the surface of the steel plate at 650°C from a temperature of minus (finish temperature -50°C) or an initial temperature of cooling. TNb (°C) = 7,500/[3.0-log10([% of Nb] x [% of C])]- 273 (1) TV (°C) = 10,800/[7.2-log10([% of V] x [% of C])]- 273 (2) TTi (°C) = 7,000/[2.8-log10([% of Ti] x [% of C])]- 273 (3)

[0022] where: [% of Nb], [% of V], [% of Ti] and [% of C] represent the content (% by mass) of Nb, V, Ti and C, respectively, in steel ; and when an element is not contained, the calculation is performed by setting the corresponding atomic symbol in the formula to 0.

[0023] In the present invention, "high strength" means the strength of 400 MPa or greater in yield strength. Furthermore, in the present invention, the "cryogenic toughness" means low temperature toughness, in other words, an energy absorbed vE-196 in a Charpy impact test at -196°C of 50 J or greater. Furthermore, in the present invention, "excellent stress corrosion cracking resistance" means a fracture stress of 500 MPa or greater when tested in accordance with a slow strain rate test method based on NACE Standard TM0111-2011 is performed by immersing in artificial seawater (18,000 ppm chloride ion concentration) at 23°C and performing a constant rate tensile test at a strain rate of 4 x 10-7 inch/second.

ADVANTAGEOUS EFFECTS OF THE INVENTION

[0024] According to the present invention, a high Mn steel plate which has excellent resistance to stress corrosion cracking and cryogenic toughness is obtained. A high Mn steel plate of the present invention greatly contributes to improved safety and lifetime of steel structures used in a cryogenic environment, such as liquefied gas storage tanks, and exerts industrially remarkable effects. In addition, the high-Mn steel plate has excellent economic efficiency without causing a decrease in productivity or an increase in manufacturing costs.

DESCRIPTION OF MODALITIES

[0025] Next, the embodiments of the present invention will be described. The present invention, however, is not limited to the following embodiments.

COMPONENT COMPOSITION

[0026] First, the component composition of a steel plate of the present invention and reasons for limiting the component composition will be described. In the present invention, to ensure excellent resistance to stress corrosion cracking, the component composition of a steel plate is specified as follows. Here, the symbol % representing component composition means % by mass unless otherwise indicated.

[0027] C: 0.20 to 0.70%

[0028] C is a cheap austenite stabilizing element and an important element to obtain austenite. To achieve such an effect, a C content of 0.20% or more is required. Meanwhile, when the content exceeds 0.70%, Cr carbide and a carbide based on Nb-, V- and/or Ti are excessively formed, thereby impairing low temperature toughness and corrosion cracking resistance. under tension. Accordingly, C is defined at 0.20 to 0.70%, preferably 0.25% or more and 0.60% or less, and more preferably 0.30% or more and 0.55% or less.

[0029] Si: 0.05 to 1.0%

[0030] Si is essential in making steel due to its action as a deoxidizing agent and even has an effect of increasing the strength of a steel plate by solid solution strengthening through dissolving in steel. To obtain such an effect, Si content of 0.05% or more is required. Meanwhile, when the content exceeds 1.0%, the weldability deteriorates and the SCC strength is also affected. Accordingly, Si is defined at 0.05 to 1.0%, preferably 0.07% or more and 0.50% or less, and more preferably 0.15% or more and 0.45% or less.

[0031] Mn: 15 to 30%

[0032] Mn is a relatively inexpensive austenite stabilizing element. In the present invention, Mn is an important element in achieving both strength and cryogenic toughness. To obtain such an effect, Mn content of 15% or more is required. Meanwhile, when the content exceeds 30%, an effect of improving cryogenic toughness levels outside and increasing alloy cost results. Furthermore, weldability and cutting properties deteriorate. Furthermore, segregation is promoted, and the occurrence of stress corrosion cracking is thereby promoted. Accordingly, Mn is set to 15 to 30%, preferably 18% or more and 28% or less, and more preferably 20% or more and 27% or less.

[0033] P: 0.028% or less

[0034] When the P content exceeds 0.028%, P is segregated to grain boundaries and becomes sites of stress corrosion cracking initiation. Consequently, the upper limit is set at 0.028% and P is desirably lowered as much as possible. P is thus defined at 0.028% or less. Meanwhile, an excessive decrease in P results in increasing refinement costs and economic disadvantages. Consequently, P is desirably set to 0.002% or greater. Preferably, P is set to 0.005% or greater and 0.024% or less.

[0035] S: 0.02% or less

[0036] Since, S impairs the low temperature toughness and ductility of a base metal, the upper limit is set at 0.02% and S is desirably decreased as much as possible. Consequently, S is set to 0.02% or less. Meanwhile, an excessive decrease in S results in increasing refinement costs and economic disadvantages. Accordingly, S is set to desirably 0.001% or more and preferably 0.002% or more. S is defined at preferably 0.018% or less and more preferably 0.010% or less.

[0037] Al: 0.01 to 0.1%

[0038] Al acts as a deoxidizing agent and is most widely used in a molten steel deoxidation process for steel plates. Furthermore, Al has an effect of suppressing the thickening of crystalline grains by fixing dissolved N in steel to form AlN. Along with such an effect, Al also has an effect of suppressing the deterioration in toughness due to a decrease in dissolved N. To obtain such effects, Al content of 0.01% or more is required. Meanwhile, when the Al content exceeds 0.1%, Al is mixed into a portion of the weld metal during welding, thereby impairing the toughness of the weld metal. Al is thus defined at 0.1% or less. Accordingly, Al is defined at 0.01 to 0.1% and preferably 0.02% or more and 0.07% or less.

[0039] Cr: 0.5 to 7.0%

[0040] Cr is an effective element to stabilize austenite through its addition in an appropriate amount and to improve the cryogenic toughness and strength of base metal. Furthermore, in the present invention, Cr is an important element that increases resistance to stress corrosion cracking through a decreased amount of hydrogen penetrating a steel plate through its effect of closely forming rust on a base metal surface. in a saltwater environment. To obtain such effects, a Cr content of 0.5% or more is required. Meanwhile, when the content exceeds 7.0%, low temperature toughness and stress corrosion cracking resistance deteriorate due to formation of Cr carbide. Cr is thus defined at 0.5 to 7.0%. Cr is defined as preferably 1.0% or more, more preferably 1.2% or more, and most preferably 2.5% or more. Meanwhile, Cr is defined at preferably 6.0% or less, more preferably 5.7% or less, and most preferably 5.5% or less.

[0041] Ni: 0.03 to 0.30%

[0042] Ni is a representative austenite stabilizing element and is an effective element for improving the cryogenic toughness and strength of base metal. Furthermore, in the present invention, Ni is an important element that increases resistance to stress corrosion cracking through a decreased amount of hydrogen penetrating a steel plate through its effect of closely forming rust on a base metal surface. in a saltwater environment. To obtain such effects, Ni content of 0.03% or more is required. Meanwhile, when the grade exceeds 0.30%, alloy costs increase, and in addition, an effect of improving corrosion cracking resistance under stress levels outside. Ni is thus defined at 0.03 to 0.30%. Preferably, Ni is defined at 0.25% or less and 0.04% or more. More preferably, Ni is defined at 0.23% or less and 0.05% or more. Furthermore, preferably, Ni is defined at 0.21% or less.

[0043] N: 0.0010 to 0.0200%

[0044] N is an austenite stabilizing element and is an effective element for improving cryogenic toughness. Furthermore, N has an effect of suppressing stress corrosion cracking as diffusible hydrogen entrapment sites through bonding with Nb, V and/or Ti to precipitate as a nitride or a carbonitride. To obtain such effects, N content of 0.0010% or more is required. Meanwhile, when the content exceeds 0.0200%, such a nitride or a carbonitride thickens, thereby impairing toughness. Consequently, N is set to 0.0010 to 0.0200%. Preferably, N is defined at 0.0020% or greater and 0.0150% or less. More preferably, N is defined at 0.0030% or greater and 0.0170% or less.

[0045] One or Two or More of Nb: 0.003 to 0.030%, V: 0.03 to 0.10%, and Ti: 0.003 to 0.040%

[0046] Nb: 0.003 to 0.030%

[0047] Nb is an element that has an effect of suppressing stress corrosion cracking through precipitation as a carbonitride (which includes a carbide), which is effective as trapping sites of diffusible hydrogen. To obtain such an effect, Nb content of 0.003% or more is required. Meanwhile, when the Nb content exceeds 0.030%, a coarse carbonitride is precipitated and becomes the source of cracking in some cases. Furthermore, coarse precipitates degrade the toughness of the base metal in some cases. Consequently, if contained, Nb is defined at 0.003 to 0.030%. Nb is defined as preferably 0.005% or more and more preferably 0.007% or more. Meanwhile, Nb is defined at preferably 0.025% or less and more preferably 0.022% or less.

[0048] V: 0.03 to 0.10%

[0049] V is an element that has an effect of suppressing stress corrosion cracking through precipitation as a carbonitride, which is effective as trapping sites of diffusible hydrogen. To obtain such an effect, a V content of 0.03% or more is required. Meanwhile, when the V content exceeds 0.10%, a coarse carbonitride is precipitated and becomes the source of cracking in some cases. Furthermore, coarse precipitates degrade the toughness of the base metal in some cases. Consequently, if contained, V is set to 0.03 to 0.10%. V is defined at preferably 0.04% or more and more preferably 0.05% or more. Meanwhile, V is set to preferably 0.09% or less, more preferably 0.08% or less, and most preferably 0.07% or less.

[0050] Ti: 0.003 to 0.040%

[0051] Ti is an element that has an effect of suppressing stress corrosion cracking through precipitation as a nitride or a carbonitride, which is effective as trapping sites of diffusible hydrogen. To obtain such an effect, Ti content of 0.003% or more is required. Meanwhile, when the Ti content exceeds 0.040%, a precipitate thickens, thereby impairing the toughness of the base metal in some cases. Furthermore, a coarse carbonitride is precipitated and becomes the source of breakage in some cases. Consequently, if contained, Ti is set at 0.003 to 0.040%. Ti is defined at preferably 0.005% or more and more preferably 0.007% or more. Meanwhile, Ti is set to preferably 0.035% or less and more preferably 0.032% or less.

[0052] The balance is iron and accidental impurities. Examples of incidental impurities include O and H, and a total of 0.01% or less is tolerable.

[0053] Furthermore, in view of the deterioration in toughness from low temperature, O and S are preferably specified as follows.

[0054] O: 0.0005 to 0.0070%

[0055] When the O content exceeds 0.0070%, coarse inclusions are formed with Al, thereby impairing low-temperature toughness. Consequently, the upper limit is set at 0.0070% and O is desirably lowered as much as possible. Preferably, O is set to 0.0060% or less. Meanwhile, an excessive decrease in O results in increasing refinement costs and economic disadvantages. Accordingly, O is set to 0.0005% or more and preferably 0.0008% or more.

[0056] O/S < 1

[0057] The balance of O and S increases resistance to stress corrosion cracking through the formation of, together with Al, Ti and Mn, an oxide, a sulfide, and a precipitated complex thereof, which effectively act as sites of diffusible hydrogen entrapment. To obtain such an effect, O/S is set to less than 1. When O/S is 1 or more, there is a risk of forming a thick oxysulfide, thereby impairing low-temperature toughness. Consequently, in the present invention, O/S is set to less than 1 to ensure low temperature toughness.

[0058] The characteristics intended to be obtained by the present invention can be obtained from the essential elements described above. In the present invention, to further improve low temperature strength and toughness, the following elements may be contained as necessary in addition to the essential elements described above.

[0059] One or two of Mo: 0.05 to 2.0% and W: 0.05 to 2.0%

[0060] Mo: 0.05 to 2.0%

[0061] Mo is a useful element for increasing the strength of a base metal and can be contained as needed. To obtain such an effect, Mo is preferably contained at 0.05% or more. Meanwhile, the content exceeding 2.0% adversely affects the toughness and cracking strength of solder in some cases. Mo is thus preferably set to 2.0% or less. Consequently, if contained, Mo is set at 0.05 to 2.0%. More preferably, Mo is defined at 0.07% or greater and 1.7% or less.

[0062] W: 0.05 to 2.0%

[0063] W is a useful element for increasing the strength of a base metal and can be contained as needed. To obtain such an effect, W is preferably contained at 0.05% or more. Meanwhile, the content exceeding 2.0% adversely affects the toughness and cracking strength of solder in some cases. W is thus preferably set to 2.0% or less. Consequently, if contained, W is set to 0.05 to 2.0%. More preferably, W is set to 0.07% or greater and 1.5% or less.

[0064] One or Two or More of Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0010 to 0.0200%

[0065] Ca: 0.0005 to 0.0050%

[0066] Ca is a useful element for controlling the morphology of an inclusion and can be contained as needed. Controlling the morphology of an inclusion here means making an elongated sulfide inclusion in a granular inclusion. Through such control of the morphology of an inclusion, ductility, toughness and resistance to sulfide stress corrosion cracking are improved. To obtain such an effect, Ca is preferably contained at 0.0005% or more. Meanwhile, when the content exceeds 0.0050%, the amount of non-metal inclusions increases. Consequently, ductility, toughness and resistance to cracking by sulfide stress corrosion cracking deteriorates greatly in some cases. In addition, economic disadvantages result in some cases. Consequently, if contained, Ca is defined at 0.0005 to 0.0050%. More preferably, Ca is defined at 0.0010% or greater and 0.0040% or less.

[0067] Mg: 0.0005 to 0.0050%

[0068] Mg is useful as an element that contributes to improving resistance to sulfide stress corrosion cracking and can be contained as needed. To obtain such an effect, Mg is preferably contained at 0.0005% or more. Meanwhile, when the content exceeds 0.0050%, the above-mentioned effect levels are outside and the effect proportional with the content cannot be expected in some cases. In addition, economic disadvantages result in some cases. Consequently, if contained, Mg is defined at 0.0005 to 0.0050%. More preferably, Mg is defined at 0.0010% or greater and 0.0040% or less.

[0069] REM: 0.0010 to 0.0200%

[0070] REM is useful as an element that contributes to improving cracking resistance by sulfide stress corrosion cracking and can be contained as needed. To obtain such an effect, REM is preferably contained at 0.0010% or more. Meanwhile, when the content exceeds 0.0200%, the above-mentioned effect levels outside and the proportional effect with the content cannot be expected in some cases. Consequently, if contained, REM is set to 0.0010 to 0.0200%. More preferably, REM is set to 0.0020% or greater and 0.0150% or less.

MICROSTRUCTURE

[0071] Next, the microstructure near a steel plate surface, which is an important requirement for a steel plate of the present invention, will be described.

[0072] The 0.5 mm Under Surface Steel Plate Microstructure That Includes Austenite as the Base Phase, in which 25% or More of Austenite, in Area Ratio, Has an Equivalent Circle Diameter of 10 μm or More and Ratio from Major Axis Aspect to Minor Axis of 3 or More.

[0073] In the present invention, the base phase of the microstructure of 0.5 mm under the steel plate surface is austenite. In the austenite, 25% or more by area ratio is austenite that has an equivalent circle diameter of 10 µm or more and an aspect ratio of 1 major axis to a minor axis of 3 or more. Because of this, strain bands within crystalline grains, in addition to grain boundaries near a steel plate surface layer, effectively act as entrapment sites for diffusible hydrogen, thereby effectively acting against cracking by stress corrosion. Consequently, suppression of stress corrosion cracking can be improved remarkably. In addition, the yield point is also improved. Preferably, the area ratio is set to 30% or more. Meanwhile, when the area ratio exceeds 95%, the strength of a steel material increases excessively, and the toughness of base metal deteriorates in some cases. Accordingly, the area ratio is set to preferably 95% or less, more preferably 94% or less, still preferably 90% or less, and still preferably 85% or less.

[0074] When an equivalent circle diameter is less than 10 μm or an aspect ratio of a major axis to a minor axis is less than 3, a desirable yield point cannot be obtained. Furthermore, strain bands within crystalline grains that effectively act as trapping sites for diffusible hydrogen cannot be obtained, and resistance to stress corrosion cracking deteriorates. Consequently, the effects described above cannot be obtained. The above-mentioned equivalent circle diameter, area ratio and aspect ratio of austenite can be measured by the methods in the Examples section described below.

[0075] In the present invention, 0.5 mm under a steel plate surface means cross sections parallel to the rolling direction at positions of 0.5 mm from the front and back surfaces of a steel plate in the thickness direction. Furthermore, in the present invention, even when the microstructure described above exists in a cross-section parallel to the rolling direction within a range of ± 5% of a 0.5 mm position under a steel plate surface, the described effects above can similarly be obtained. Consequently, in the present invention, 0.5 mm under a steel plate surface means that the microstructure described above exists in a cross section parallel to the rolling direction anywhere within a range of ± 5% of positions of 0.5 mm from the front and back surfaces of the steel plate in the thickness direction. Front and back surfaces refer to not only intact surfaces of a finished product, but also steel plate surfaces that have been treated such that a cumulative grade of a crystal can be measured. For example, when the outermost surfaces of a steel plate are covered with fouling, the surfaces after the fouling has been removed are meant.

[0076] The microstructure of 0.5 mm under the surface of steel plate which still includes, within the microstructure, a total number of 2x102/mm2 or more of carbide, nitride and carbonitride which contains one or two or more of Nb, V, and Ti and Which Have an Equivalent Circle Diameter of 0.01 to 0.5 µm.

[0077] The state of existence of a carbide, a nitride and a carbonitride (hereinafter referred to as precipitates based on Nb-, V- and/or Ti) containing one or two or more of Nb, V and Ti in the microstructure of 0.5 mm under a steel plate surface of the present invention will be described. Here, a carbide, a nitride, and a carbonitride containing one or two or more of Nb, V, and Ti refers to: a carbide containing one or two or more of Nb, V, and Ti; a nitride containing one or two or more of Nb, V and Ti; and a carbonitride containing one or two or more of Nb, V and Ti.

[0078] The particle size of precipitates based on Nb-, V- and/or Ti is set to 0.01 to 0.5 μm in equivalent circle diameter. When the particle size is smaller than 0.01 μm, an effect of suppressing the cracking of hydrogen embrittlement as places of entrapment of diffusible hydrogen levels outside. Furthermore, controlling the particle size to be less than 0.01 µm in actual manufacturing results in excessively increased manufacturing burden and increased manufacturing costs. Meanwhile, when the particle size exceeds 0.5μm, the low temperature toughness deteriorates. Furthermore, it is impossible to obtain an effect of suppressing hydrogen embrittlement cracking as diffusible hydrogen trapping sites. Preferably, the particle size is set to 0.03 µm or more and 0.4 µm or less.

[0079] When the total number of precipitates based on Nb-, V- and/or Ti that have the particle size described above is less than 2x102/mm2 in the microstructure of 0.5 mm under a steel plate surface , precipitates that act as trapping sites for diffusible hydrogen are insufficient. Consequently, an effect of suppressing hydrogen embrittlement cracking as diffusible hydrogen trapping sites cannot be obtained. Accordingly, the total number is set to 2x102/mm2 or more and preferably 5x102/mm2 or more. The aforementioned number density and equivalent circle diameter of the precipitates based on Nb-, V- and/or Ti can be measured by the methods in the Examples section described below.

[0080] When martensite and other microstructures coexist with austenite in the 0.5 mm microstructure under a steel plate surface, the low-temperature toughness deteriorates. Consequently, austenite is defined at 90% or more. In view of low-temperature deterioration in toughness, the area ratio of martensite and other microstructures is preferably small. Martensite and other microstructures mentioned above herein refer to martensite, bainite, ferrite and pearlite. When martensite and other microstructures coexist with austenite, the total area ratio of each microstructure is desirably set to 10% or less based on the entire steel plate.

MANUFACTURING CONDITIONS

[0081] Next, a manufacturing method for a steel plate of the present invention will be described. A steel plate according to the present invention is suitable for a high Mn steel plate that has a thickness of 4 mm or more.

[0082] A steel plate of the present invention is obtained by: heating steel having the component composition described above to a temperature range of (Tx-50) °C or greater and (Tx+200) °C or lowest as a steel surface temperature for any one or more of Tx (°C) defined by any one of formulas (1) to (3) when Tx (x = Nb, V or Ti) is defined at a temperature represented by any one of formula (1) to (3) described below; hot rolling at a finishing temperature of 750°C or greater and 1000°C or less to produce a steel plate; and subsequently cooling at an average cooling rate of 1.0°C/s or more on the surface of the steel plate at 650°C from a temperature of minus (finish temperature -50°C) or an initial temperature of cooling.

[0083] Next, the details will be described. In the description, the symbol “°C” referring to a temperature refers to a temperature on a steel plate surface or a steel surface.

[0084] In a high-Mn steel plate according to the present invention, molten steel having the component composition described above can be refined by a publicly known refining method, such as using a converter or an electric furnace . Additionally, secondary refinement can be performed in a vacuum degasser. Subsequently, steel, such as a sheet of a predetermined size, is preferably formed by a continuous casting method or a publicly known casting method, such as an ingot casting/plating method.

[0085] Sheet Metal After Casting: Steel Heating Obtained, Without Cooling to Room Temperature or After Cooling to Room Temperature, at Temperature Range of (Tx-50) °C or greater and (Tx+200) °C or less than Steel surface temperature for any one of or More than Tx (°C) defined by any one of formulas (1) to (3) When Tx (x = Nb, V or Ti) is defined the Temperature Represented by any one from formula (1) to (3) TNb (°C) = 7,500/[3.0-log10([% of Nb] x [% of C])]- 273 (1) TV (°C) = 10,800/ [7.2-log10([% of V] x [% of C])]- 273 (2) TTi (°C) = 7,000/[2.8-log10([% of Ti] x [% of C ])]- 273 (3)

[0086] where: [% of Nb], [% of V], [% of Ti] and [% of C] represent the content (% by mass) of Nb, V, Ti and C, respectively, in steel ; and when an element is not contained, the calculation is performed by setting the corresponding atomic symbol in the formula to 0.

[0087] When the heating temperature is less than (Tx-50)°C, creep strength in hot rolling increases while reduction per pass decreases. Consequently, an increase in the number of rolling passes results in low rolling efficiency. At the same time, casting defects within steel (plate) cannot be pressure bonded in some cases. Furthermore, crystals containing Nb-, V- and Ti that were unevenly formed within steel in the refinement stage remain within a steel plate even after the end of rolling. Consequently, the desirable Nb-, V-, and Ti-containing precipitates cannot be obtained and the resistance to stress corrosion cracking deteriorates.

[0088] Meanwhile, when the heating temperature exceeds (Tx+200) °C, surface scratches readily arise due to scale formed during heating, thereby increasing a repair load after lamination. Furthermore, a steel surface is excessively decarburized, and a steel plate surface after rolling thus becomes martensite. Consequently, flexibility and/or hydrogen embrittlement strength deteriorates. Furthermore, due to coarsening of austenite grains, the intended microstructure cannot be achieved.

[0089] Consequently, the steel heating temperature is set to (Tx-50) °C or higher and (Tx+200) °C or lower. Preferably, the heating temperature is set to (Tx-30) °C or greater and (Tx+180) °C or less. In case of direct rolling, hot rolling is started while steel is at (Tx-50) °C or higher and (Tx+200) °C or lower.

[0090] Here, the indication of “heating to a temperature range of (Tx-50) °C or greater and (Tx+200) °C or less as a steel surface temperature for any one or more of Tx ( °C) defined by any one of formulas (1) to (3) when Tx (x = Nb, V or Ti) is defined at a temperature represented by any one of formulas (1) to (3)” of the present invention means that when the component composition described above contains two elements of Nb and V, for example, the heating temperature can satisfy one or more of (TNb-50) °C or greater and (TNb+200) °C or less, or (TV-50) °C or higher and (TV+200) °C or lower. In other words, any of the heating temperatures can be selected.

[0091] Hot Rolling: Steel Plate Having Desirable Thickness Is Obtained by Adjusting the Finishing Temperature to 750°C or Higher and 1,000°C or Lower in Finishing Rolling After Roughness Increase.

[0092] When a finishing temperature in hot rolling exceeds 1000°C, the recrystallization of austenite near a steel plate surface readily progresses and the desirable microstructure cannot be obtained. Consequently, resistance to stress corrosion cracking deteriorates. Meanwhile, when a finishing temperature is set at less than 750°C, hot creep strength increases excessively, thereby increasing a load on a rolling mill. Furthermore, low lamination efficiency and result in increased manufacturing costs. Accordingly, a hot rolling finish temperature is defined as 750°C or greater and 1000°C or less, preferably 800°C or greater and 950°C or less, and more preferably 940°C or less.

[0093] Cumulative Reduction of 10% or More and 50% or Less in a Temperature Range of 850°C or Greater and (Tx-50) °C or Less in Finish Lamination (Preferred Condition).

[0094] When a cumulative reduction is less than 10% in the temperature range of 850°C or greater and (Tx-50) °C or less, there is a risk of failure to obtain the target microstructure. Meanwhile, when the cumulative reduction exceeds 50%, the rolling efficiency decreases. Furthermore, there is a risk that strength increases excessively and low temperature toughness deteriorates. Here, the cumulative reduction is a total reduction obtained by adding a reduction in each rolling pass in the temperature range of 850°C or greater and (Tx-50) °C or less in finish rolling.

[0095] Cumulative Reduction of 5% or More and 60% or Less in Non-Recrystallization Region (960°C or Less) in Finish Lamination (Most Preferential Condition)

[0096] When a cumulative reduction is less than 5% in the non-recrystallization region, there is a risk of failure to achieve the target strength. Meanwhile, when a cumulative shrinkage exceeds 60%, there is a risk that the yield point increases excessively and the low-temperature toughness deteriorates. Here, the cumulative reduction is a total reduction obtained by adding a reduction in each rolling pass in the non-recrystallization region in finishing rolling.

[0097] After the End of Finish Rolling, Cooling at an Average Cooling Rate of 1.0°C/s or More on a Steel Plate Surface at 650°C From a Temperature of less than (Finishing Temperature -50° C) or Initial Cooling Temperature.

[0098] When an average cooling rate on a steel plate surface is less than 1.0°C/s, a carbide thickens due to retention at a high temperature for a long time, thereby decreasing strength. In addition, Cr carbide is formed, thereby impairing toughness and resistance to stress corrosion cracking. Accordingly, the average cooling rate is set to preferably 1.0°C/s or more and more preferably 2.0°C/s or more. Meanwhile, when the average cooling rate exceeds 150.0°C/s, it becomes difficult to maintain the shape of a steel plate. Accordingly, the average cooling rate is defined as preferably 150.0°C/s or less, more preferably 120.0°C/s or less, and most preferably 100.0°C/s or less. Here, the average cooling rate is an average cooling rate at 650° from a temperature of less than (finish temperature -50°C) or an initial cooling temperature after the end of finish rolling.

[0099] In the present invention, it was recently found that controlling an average cooling rate in cooling is effective to suppress Cr carbide precipitation during cooling and thereby improve resistance to stress corrosion cracking.

[00100] Here, an average cooling rate in the temperature range from a finishing temperature to (finishing temperature -50°C) is not particularly specified, but is preferably 1.0°C/s or less since the formation of precipitates based on Nb-, V- and/or Ti can be promoted. Furthermore, an average cooling rate at less than 650°C is not particularly specified, but is preferably defined as less than 100.0°C/s from a point of view of preventing deformation of a steel plate. and more preferably 80.0°C/s or less.

EXAMPLES

[00101] Next, the present invention will be described in more detail with Examples. The present invention, however, is not limited to the following Examples.

[00102] Steel sheets (plate thickness: 250 to 300 mm) were prepared to have various component compositions shown in Table 1-1 and Table 1-2 by a converter/shell, continuous refining/casting method. The steel sheets were heated to (Tx-50) °C or higher and (Tx+200) °C or lower (x = Nb, V or Ti), then hot rolled under manufacturing conditions shown in Table 2-1 and Table 2-2, and cooled under the manufacturing conditions shown in Table 2-1 and Table 2-2. Here, (Tx-50) °C and (Tx+200) °C for Nb, V or V are each shown in Table 1-1 and Table 12.

[00103] The hot-rolled steel plates from 12 mm to 80 mm thick obtained were subjected to a microstructure examination, a base metal tensile test, a base metal toughness test, and a cracking test by stress corrosion in the following manner.

(1) MICROSTRUCTURE

[00104] In the microstructure examination, a sample for microstructure observation was taken from each of the steel plates obtained in a cross section parallel to the rolling direction at a position of 0.5 mm under the surface in the thickness direction , marked with an aqueous solution of sodium pyrosulfite (10 g of Na2S2O5+95 ml of aqueous solution), and visually represented by the optical microscopic structure in five fields of view at a magnification of 500x. Subsequently, an austenite area ratio, an equivalent circle diameter, and an aspect ratio were obtained from each of the microstructure images obtained using an image analyzer.

AUSTENITE AREA RATIO

[00105] The austenite area ratio was obtained as an austenite area ratio of 10 μm or more to the total austenite area by performing austenite marking, microstructure imaging at 500x magnification, tracing grain boundaries of austenite, and performing image analysis.

AUSTENITE EQUIVALENT CIRCLE DIAMETER

[00106] As for the austenite grain size, in other words, the austenite equivalent circle diameter, the individual austenite areas were first determined through image analysis of the above-mentioned microstructure images. The equivalent circle diameter was then calculated from individual areas.

AUSTENITE GRAIN ASPECT RATIO

[00107] The aspect ratio of austenite grains was calculated as a ratio of the largest diameter (major axis) to the largest width orthogonal to the major axis (minor axis) for each austenite grain through observation under an optical microscope of the microstructure in which austenite grain boundaries have been exposed by the aforementioned marking.

EQUIVALENT CIRCLE DIAMETER OF PRECIPITATES BASED ON Nb-, V- AND/OR Ti

[00108] In examining the equivalent circle diameter of precipitates based on Nb-, V- and/or Ti, ten fields of view were visually represented at a magnification of 50,000x under a transmission electron microscope in a cross section parallel to the rolling direction at a position 0.5 mm under the surface of each steel plate in the thickness direction, and the area of each precipitate based on Nb-, V- and/or Ti was determined through image analysis of these microstructure images. The equivalent circle diameter of precipitates based on Nb-, V- and/or Ti was calculated from each area.

NUMERICAL DENSITY OF PRECIPITATES BASED ON Nb-, V- AND/OR Ti

[00109] In examining the numerical density of precipitates based on Nb-, V- and/or Ti, ten fields of view were visually represented under a transmission electron microscope at a magnification of 50,000x in cross section parallel to the lamination direction at a position 0.5 mm under the surface of each steel plate in the thickness direction, the number of precipitates based on Nb-, V- and/or Ti that have an equivalent circle diameter of 0.01 to 0 .5 µm was counted per 1 mm2, and a total precipitate density number based on Nb-, V- and/or Ti was obtained.

(2) BASE METAL TRACTION CHARACTERISTICS

[00110] The tensile characteristics were examined by taking JIS No 5 tensile samples from each of the steel plates obtained and performing a tensile test according to JIS Z 2241 (1998). In the present invention, a sample having a yield strength of 400 MPa or greater is evaluated as excellent base metal tensile characteristics (within the scope of the present invention). Samples with excellent tensile characteristics of base metal of the present invention had a tensile strength of 800 MPa or more and total elongation of 30% or more.

(3) BASE METAL TENACITY

[00111] The base metal toughness was evaluated by: taking Charpy V-notch samples according to JIS Z 2202 (1998) in a direction perpendicular to the rolling direction in a 1/4 thickness position for each plate of steel having a thickness of more than 20 mm or in a position of 1/2 thickness for each steel plate having a thickness of 20 mm or less; perform a Charpy impact test for three samples for each steel plate in accordance with JIS Z 2242 (1998); and obtain an absorbed energy at -196°C. In the present invention, a steel plate having an average absorbed energy (vE-196) of three samples of 50 J or greater is evaluated as excellent base metal toughness (within the scope of the present invention). More preferably, the average absorbed energy (vE-196) is 100 J or greater.

(4) PROPERTY VOLTAGE CORROSION CRACKING

[00112] A stress corrosion cracking test was performed according to a slow strain rate test method based on NACE Standard TM0111-2011. A test piece having a notched Type A round bar shape was used. The test piece was immersed in artificial seawater (18,000 ppm chloride ion concentration) at 23°C and subjected to a constant rate tensile test at a strain rate of 4x10-7 inch/second. In the present invention, a test piece having a fracture stress of 500 MPa or greater is rated as having excellent resistance to stress corrosion cracking (within the scope of the present invention). More preferably, a fracture stress is 600 MPa or greater.

Nota 1: Sublinhados indica o exterior do escopo da presente invenção TNb(°C)=7.500/{3,0-logio([% de Nb]x[% de C])}-273---(1) Tv(°C)=10.800/{7,2-logio([% de V]x[% de C])}-273---(2) TTi(°C)=7.000/{2,8-logio([% de Ti]x[% de C])}-273---(3) Nota 2: em que: [% de Nb], [% de V], [% de Ti] e [% de C] representam o conteúdo (% em massa) de Nb, V, Ti e C, respectivamente, em aço; e quando um elemento não está contido, o cálculo é realizado ajustando-se o símbolo atômico correspondente na fórmula para 0. TABELA 1-2

Nota 1: Sublinhados indicam o exterior do escopo da presente invenção TNb(°C)=7.500/{3,0- logio ([% de Nb]x[% de C])}-273---(1) TV(°C)=10.800/{7,2- logio ([% de V]x[% de C])}-273-(2) Nota 2: TTÍ(°C)=7.000/{2,8- logio ([% de Ti]x[% de C])}-273---(3) em que: [% de Nb], [% de V], [% de Ti] e [% de C] representam o conteúdo (% em massa) de Nb, V, Ti e C em aço, respectivamente; e quando um elemento não está contido, o cálculo é realizado ajustando-se o símbolo atômico correspondente na fórmula para 0. TABELA 2-1

Nota: sublinhados indicam o exterior do escopo da presente invenção TABELA 2-2

Nota: Sublinhados indicam o exterior do escopo da presente invenção TABELA 3-1

Nota: sublinhados indicam o exterior do escopo da presente invenção TABELA 3-2

Nota: sublinhados indicam o exterior do escopo da presente invenção[00113] The results obtained as above are shown in Table 3-1 and Table 3-2. TABLE 1-1

Note 1: Underscores indicate outside the scope of the present invention TNb(°C)=7,500/{3,0-logio([% of Nb]x[% of C])}-273---(1) Tv( °C)=10,800/{7,2-logion([% of V]x[% of C])}-273---(2) TTi(°C)=7,000/{2,8-logion([ % of Ti]x[% of C])}-273---(3) Note 2: where: [% of Nb], [% of V], [% of Ti] and [% of C] represent the content (% by mass) of Nb, V, Ti and C, respectively, in steel; and when an element is not contained, the calculation is performed by setting the corresponding atomic symbol in the formula to 0. TABLE 1-2

Note 1: Underscores indicate outside the scope of the present invention TNb(°C)=7,500/{3,0- logio ([% of Nb]x[% of C])}-273---(1) TV( °C)=10,800/{7.2- logio ([% of V]x[% of C])}-273-(2) Note 2: TTÍ(°C)=7,000/{2.8- logio ( [% Ti]x[% C])}-273---(3) where: [% Nb], [% V], [% Ti] and [% C] represent the content (% by mass) of Nb, V, Ti and C in steel, respectively; and when an element is not contained, the calculation is performed by setting the corresponding atomic symbol in the formula to 0. TABLE 2-1

Note: underscores indicate outside the scope of the present invention TABLE 2-2

Note: Underscores indicate the outside scope of the present invention TABLE 3-1

Note: underscores indicate outside the scope of the present invention TABLE 3-2

Note: underscores indicate the outside scope of the present invention

[00114] The Examples have been confirmed to meet the above-mentioned target performance (base metal yield strength of 400 MPa or greater, low temperature toughness of 50 J or greater as average absorbed energy (vE-196), the stress corrosion cracking strength of 500 MPa or greater as stress fracture). Meanwhile, Comparative Examples outside the scope of the present invention may not satisfy the above-mentioned target performance in any one or more of base metal strength, low temperature toughness, and resistance to stress corrosion cracking. In Table 3-1 and Table 3-2, steel plates No. 12 and 36 of Comparative Examples had, by area ratio, 70% austenite with an average equivalent circle diameter of 10 µm or more and an aspect ratio from a major axis to a minor axis of 3 or more. This is because stable austenite is scarce, but unstable austenite predominates since C in the component composition is beyond the scope of the present invention.

Claims (3)

1. High Mn steel plate, characterized in that it has a component composition consisting of, wt%, C: 0.20 to 0.70%, Si: 0.05 to 1.0%, Mn: 15 to 30%, P: 0.028% or less, S: 0.02% or less, Al: 0.01 to 0.1%, Cr: 0.5 to 7.0%, Ni: 0.03 to 0 .30%, N: 0.0010 to 0.0200%, and one or two or more of Nb: 0.003 to 0.030%, V: 0.03 to 0.10%, Ti: 0.003 to 0.040%, and optionally one element in at least one group selected from the following group A or B: Group A: one or two, % by mass, selected from Mo: 0.05 to 2.0%, and W: 0.05 to 2 .0%, Group B: one or two or more, % by mass, selected from Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0010 to 0.0200%, with the balance being Fe and incidental impurities, where: a 0.5 mm microstructure under a steel plate surface includes austenite as a base phase; and 25% or more of the austenite, by area ratio, has an equivalent circle diameter of 10 µm or more and an aspect ratio of 1 major axis to a minor axis of 3 or more. 2. High Mn steel plate, according to claim 1, characterized by the fact that the microstructure of 0.5 mm under the surface of the steel plate still includes a total number of 2x102/mm2 or more than one carbide, a nitride and a carbonitride, the carbide, the nitride, and the carbonitride containing one or two or more of Nb, V, and Ti and having an equivalent circle diameter of 0.01 to 0.5 µm. 3. Manufacturing method for a high Mn steel plate, characterized in that it comprises: heating the steel having the component composition, as defined in claim 1 or 2, to a temperature range of (Tx-50) °C or greater and (Tx+200) °C or less as a steel surface temperature for any one or more of Tx (°C) defined by any one of formulas (1) to (3) when Tx (x = Nb, V or Ti) is defined at a temperature represented by any one of formulas (1) to (3); hot rolling at a finishing temperature of 750°C or greater and 1000°C or less to produce a steel plate; and subsequently cool at an average cooling rate of 1.0°C/s or more on the surface of the steel plate at 650°C from a temperature of minus (finish temperature -50°C) or an initial temperature of cooling, TNb (°C) = 7,500/[3.0-log10([% of Nb] x [% of C])]- 273 (1) TV (°C) = 10,800/[7.2-log10 ([% of V] x [% of C])]- 273 (2) TTi (°C) = 7,000/[2.8-log10([% of Ti] x [% of C])]- 273 ( 3) in which: [% of Nb], [% of V], [% of Ti] and [% of C] represent the content (% by mass) of Nb, V, Ti and C, respectively, in steel; and when an element is not contained, the calculation is performed by setting the corresponding atomic symbol in the formula to 0.